Phosphorylation and steroid hormone action

Role of Phosphorylation in the Modulation of the Glucocorticoid Receptor's Intrinsically Disordered Domain

Protein phosphorylation often switches cellular activity from one state to another, and this post-translational modification plays an important role in gene regulation by the nuclear hormone receptor superfamily, including the glucocorticoid receptor (GR). Cell signaling pathways that regulate phosphorylation of the GR are important determinants of GR actions, including lymphoid cell apoptosis, DNA binding, and interaction with coregulatory proteins. All major functionally important phosphorylation sites in the human GR are located in its N-terminal domain (NTD), which possesses a powerful transactivation domain, AF1. The GR NTD exists as an intrinsically disordered protein (IDP) and undergoes disorder-order transition for AF1’s efficient interaction with several coregulatory proteins and subsequent AF1-mediated GR activity. It has been reported that GR’s NTD/AF1 undergoes such disorder-order transition following site-specific phosphorylation. This review provides currently available information regarding the role of GR phosphorylation in its action and highlights the possible underlying mechanisms of action.

Molecular regulation of hypothalamus-pituitary-gonads axis in males

The hypothalamic-pituitary-gonadal axis (HPG) plays vital roles in reproduction and steroid hormone production in both sexes. The focus of this review is upon gene structures, receptor structures and the signaling pathways of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The hormones' functions in reproduction as well as consequences resulting from mutations are also summarized. Specific characteristics of hormones such as the pulsatile secretions of GnRH are also covered. The different regulators of the HPG axis are introduced including kisspeptin, activin, inhibin, follistatin, androgens and estrogen. This review includes not only their basic information, but also their unique function in the HPG axis. Here we view the HPG axis as a whole, so relations between ligands and receptors are well described crossing different levels of the HPG axis. Hormone interactions and transformations are also considered. The major information of this article is depicted in three figures summarizing the current discoveries on the HPG axis. This article systematically introduces the basic knowledge of the HPG axis and provides information of the current advances relating to reproductive hormones.

Progesterone signaling mechanisms in brain and behavior

Steroid hormone, progesterone, modulates neuroendocrine functions in the central nervous system resulting in alterations in physiology and behavior. These neuronal effects are mediated primarily by intracellular progestin receptors (PRs) in the steroid-sensitive neurons, resulting in transcription-dependent genomic actions (classical mechanism). In addition to progesterone, intracellular PRs can also be activated in a "ligand-independent" manner by neurotransmitters, peptide growth factors, cyclic nucleotides, and neurosteroids. Recent studies indicate that rapid, non-classical progesterone actions involving cytoplasmic kinase signaling and/or extranuclear PRs can result in both transcription-independent and transcription-dependent actions. Cross-talk between extranuclear and classical intracellular signaling pathways promotes progesterone-dependent behavior in mammals. This review focuses on the mechanisms by which progesterone-initiated signaling mechanisms converge with PRs in the brain to modulate reproductive behavior in female rodents.

Differential regulation of the transcriptional activity of the glucocorticoid receptor through site-specific phosphorylation

Post-translational modifications such as phosphorylation are known to play an important role in the gene regulation by the transcription factors including the nuclear hormone receptor superfamily of which the glucocorticoid receptor (GR) is a member. Protein phosphorylation often switches cellular activity from one state to another. Like many other transcription factors, the GR is a phosphoprotein, and phosphorylation plays an important role in the regulation of GR activity. Cell signaling pathways that regulate phosphorylation of the GR and its associated proteins are important determinants of GR function under various physiological conditions. While the role of many phosphorylation sites in the GR is still not fully understood, the role of others is clearer. Several aspects of transcription factor function, including DNA binding affinity, interaction of transactivation domains with the transcription initiation complex, and shuttling between the cytoplasmic compartments, have all been linked to site-specific phosphorylation. All major phosphorylation sites in the human GR are located in the N-terminal domain including the major transactivation domain, AF1. Available literature clearly indicates that many of these potential phosphorylation sites are substrates for multiple kinases, suggesting the potential for a very complex regulatory network. Phosphorylated GR interacts favorably with critical coregulatory proteins and subsequently enhances transcriptional activity. In addition, the activities and specificities of coregulators may be subject to similar regulation by phosphorylation. Regulation of the GR activity due to phosphorylation appears to be site-specific and dependent upon specific cell signaling cascade. Taken together, site-specific phosphorylation and related kinase pathways play an important role in the action of the GR, and more precise mechanistic information will lead to fuller understanding of the complex nature of gene regulation by the GR- and related transcription factors. This review provides currently available information regarding the role of GR phosphorylation in its action, and highlights the possible underlying mechanisms of action.

Activation of progestin receptors in female reproductive behavior: Interactions with neurotransmitters

The steroid hormone, progesterone (P), modulates neuroendocrine functions in the central nervous system resulting in alterations in physiology and reproductive behavior in female mammals. A wide body of evidence indicates that these neural effects of P are predominantly mediated via their intracellular progestin receptors (PRs) functioning as "ligand-dependent" transcription factors in the steroid-sensitive neurons regulating genes and genomic networks. In addition to P, intracellular PRs can be activated by neurotransmitters, growth factors and cyclic nucleotides in a ligand-independent manner via crosstalk and convergence of pathways. Furthermore, recent studies indicate that rapid signaling events associated with membrane PRs and/or extra-nuclear, cytoplasmic PRs converge with classical PR activated pathways in neuroendocrine regulation of female reproductive behavior. The molecular mechanisms, by which multiple signaling pathways converge on PRs to modulate PR-dependent female reproductive behavior, are discussed in this review.

Identification of four novel phosphorylation sites in estrogen receptor alpha: impact on receptor-dependent gene expression and phosphorylation by protein kinase CK2

Background

Estrogen receptor α (ERα) phosphorylation is important for estrogen-dependent transcription of ER-dependent genes, ligand-independent receptor activation and endocrine therapy response in breast cancer. However ERα phosphorylation at the previously identified sites does not fully account for these receptor functions. To determine if additional ERα phosphorylation sites exist, COS-1 cells expressing human ERα were labeled with [³²P]H₃PO₄ in vivo and ERα tryptic phosphopeptides were isolated to identify phosphorylation sites.

Results

Previously uncharacterized phosphorylation sites at serines 46/47, 282, 294, and 559 were identified by manual Edman degradation and phosphoamino acid analysis and confirmed by mutagenesis and phospho-specific antibodies. Antibodies detected phosphorylation of endogenous ERα in MCF-7, MCF-7-LCC2, and Ishikawa cancer cell lines by immunoblot. Mutation of Ser-282 and Ser-559 to alanine (S282A, S559A) resulted in ligand independent activation of ERα as determined by both ERE-driven reporter gene assays and endogenous pS2 gene expression in transiently transfected HeLa cells. Mutation of Ser-46/47 or Ser-294 to alanine markedly reduced estradiol dependent reporter activation. Additionally protein kinase CK2 was identified as a kinase that phosphorylated ERα at S282 and S559 using motif analysis, in vitro kinase assays, and incubation of cells with CK2 kinase inhibitor.

Conclusion

These novel ERα phosphorylation sites represent new means for modulation of ERα activity. S559 represents the first phosphorylation site identified in the extreme C-terminus (F domain) of a steroid receptor.

Gene regulation by the glucocorticoid receptor: structure:function relationship

The glucocorticoid receptor (GR) belongs to the superfamily of ligand-activated transcription factors, the nuclear hormone receptors. Like other members of the family, the GR possesses a modular structure consisting of three major domains-the N-terminal (NTD), DNA binding (DBD), and ligand binding (LBD). Although the structures of independently expressed GR DBD and LBD are known, the structures of the NTD and of full-length GR are lacking. Both DBD and LBD possess overall globular structures. Not much is known about the structure of the NTD, which contains the powerful AF1/tau1/enh2 transactivation region. Several studies have shown that AF1 region is mostly unstructured and that it can acquire folded functional conformation under certain potentially physiological conditions, namely in the presence of osmolytes, when the GR DBD is bound to glucocorticoid response element (GRE), and when AF1 binds other transcription factor proteins. These conditions are discussed here. The functions of the GR will be fully understood only when its working three-dimensional structure is known. Based on the available data, we propose a model to explain data which are not adequately accounted for in the classical models of GR action. In this review, we summarize and discuss current information on the structure of the GR in the context of its functional aspects, such as protein:DNA and protein:protein interactions. Because of the close similarities in modular organization among the members of the nuclear hormone receptors, the principles discussed here for the GR should be applicable to many other receptors in the family as well.

Conditionally immortal ovarian cell lines for investigating the influence of ovarian stroma on the estrogen sensitivity and tumorigenicity of ovarian surface epithelial cells

The tendency of the ovarian surface epithelium (OSE) to undergo metaplastic and morphogenetic changes during the life cycle, at variance with the adjacent peritoneal mesothelial cells, suggests that its biology may be regulated by underlying ovarian stromal cues. However, little is known about the role that the ovarian stroma plays in the pathobiology of the OSE, largely because of the lack of a suitable in vitro model. Here, we describe the establishment and characterization of conditionally immortalized ovarian stromal and surface epithelial cell lines from H-2K(b)-tsA58 transgenic mice that carry the thermolabile mutant of SV-40 large T antigen under the control of an interferon-gamma (IFN-gamma)-inducible promoter. These cells express functional T antigens, grow continuously under permissive conditions at 33 degrees C in the presence of IFN-gamma, and stop dividing when the activity and expression of the tumor antigen is suppressed by restrictive conditions without IFN-gamma at 39 degrees C. Morphological, immunohistochemical, and ultrastructural analyses show that conditionally immortal OSE cells form cobblestone-like monolayers, express cytokeratin and vimentin, contain several microvilli, and develop tight junctions, whereas stromal cells are spindle-like, express vimentin but not cytokeratin, and contain rare microvilli, thus exhibiting epithelial and stromal phenotypes, respectively. At variance with the reported behavior of rat epithelial cells, conditionally immortal mouse epithelial cells are not spontaneously transformed after continuous culture in vitro. More importantly, conditioned media from stromal cells cultured under permissive conditions increase the specific activity of the endogenous estrogen receptor in BG-1 human ovarian epithelial cancer cells and promote these cells' anchorage-independent growth, suggesting the paracrine influence of a stromal factor. In addition, stromal cells cultured under restrictive conditions retain this growth-stimulatory activity, which, therefore, appears to be independent of T antigen expression. These established cell lines should provide a useful in vitro model system for studying the role of cellular interactions in OSE cell growth and tumorigenesis.

Antagonism between PTEN/MMAC1/TEP-1 and androgen receptor in growth and apoptosis of prostatic cancer cells

PTEN/MMAC1/TEP-1 (PTEN) tumor suppressor and androgen receptor play important roles in prostatic tumorigenesis by exerting opposite effects on homeostasis of prostatic epithelium. Here, we describe a mutual repression and selective dominance between PTEN and the androgen receptor (AR) in the growth and the apoptosis of prostatic cancer cells. On the one hand, PTEN and an inhibitor of phosphoinositide 3-kinase repressed the transcriptional activity of the AR as well as androgen-induced cell proliferation and production of prostate-specific antigen. On the other hand, androgens protected prostate cancer cells from PTEN-induced apoptosis in an AR-dependent manner. Whereas the repression of the transcriptional activity of the AR by PTEN is likely to involve the down-regulation of AKT, androgens protected prostate cancer cells from PTEN-induced apoptosis without an effect on AKT activity, demonstrating a differential involvement of AKT in the interaction between PTEN and the AR. Our data suggest that the loss of PTEN function may induce tumorigenesis through unopposed activity of the AR as well as contribute to the resistance of prostate cancers to androgen ablation therapy.

Genomic and membrane actions of progesterone: implications for reproductive physiology and behavior

Progesterone, produced by the ovaries and adrenal glands, regulates reproductive behavior and the surge of luteinizing hormone which precedes ovulation by acting on neurons located in different parts of the hypothalamus. The study of the activation of these reproductive functions in female rats has allowed to explore the different mechanisms of progesterone action in the brain. It has allowed to demonstrate that new actions of the hormone, which have been observed in particular in vitro systems, are also operational in vivo, and may thus be biologically relevant. This mainly concerns the direct actions of progesterone on receptors of neurotransmitters such as oxytocin and GABA. Activation of the progesterone receptor in the absence of ligand by phosphorylation may also play a role.

Ecdysteroid receptor and ultraspiracle from Chironomus tentans (Insecta) are phosphoproteins and are regulated differently by molting hormone

Three different isotypes of the ecdysteroid receptor (cEcR) (66, 68 and 70 kDa) and several molecular variants of the dimerization partner "ultraspiracle" (cUSP) (58-77 kDa) can be separated electrophoretically in homogenates of the epithelial cell line from Chironomus tentans. After phosphatase treatment the bands with the lowest electrophoretic mobility disappear in both cases. Phosphorylation occurs exclusively at ser/thr in EcR and USP. Binding studies with 3H-ponasterone A using 0.4 M NaCl extracts revealed two classes of high-affinity binding (KD1 = 0.47 and KD2 = 7.2 nM) competable either with 20-OH-ecdysone or muristerone A. At least KD2 and Bmax2 are unchanged after dephosphorylation. In hormonally naive cells a considerable part of EcR and USP is already present in nuclei. The phosphorylation pattern of both transcription factors is the same in cytosol and nuclear fractions. Incubation with 20-OH-ecdysone (1 microM, up to 4 days) does not alter the extent and mode of phosphorylation of EcR, although EcR concentration increases. In contrast USP concentration remains constant but phosphorylation is enhanced.

Hormone-activated phosphorylation of human beta1 thyroid hormone nuclear receptor

To understand the role of phosphorylation in the hormone-dependent transcriptional activation of thyroid hormone receptors (TRs), the present study evaluated the effect of the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3) on the phosphorylation of TR, human subtype beta1 (h-TRbeta1). The extent of phosphorylation was compared in cells cultured in T3-depleted (Td) or T3-supplemented medium (Td + T3). T3 was found to activate phosphorylation of h-TRbeta1 approximately threefold. Taking into account the T3-induced fourfold downregulation in the expression of h-TRbeta1 in the same period, the specific T3-activated phosphorylation was increased approximately twelvefold. Phosphoamino acid analysis indicates that the phosphorylation of serine and threonine in a ratio of approximately 10:1 was increased approximately threefold by T3. Comparison of the [32P]-labeled tryptic maps of h-TRbeta1 phosphorylated in cells cultured in Td medium or Td + T3 medium indicates that the latter had fewer fragments and changes of intensities in several common fragments, indicating that the phosphorylation sites activated by T3-treatment differed from those of basal phosphorylation. Partial V8 and chymotrypic proteolysis indicates that h-TRbeta1 phosphorylated in cells cultured in Td + T3 medium was more resistant to proteolysis. These results indicate that T3-activated phosphorylation altered the protease susceptibility of h-TRbeta1 that could reflect structural changes in h-TRbeta1. These results raise the possibility that T3-activated phosphorylation may play an important role in transcriptional activation of h-TRbeta1.

Phosphorylation of Ser211 in the chicken progesterone receptor modulates its transcriptional activity

The chicken progesterone receptor has been shown to be phosphorylated in vivo at four major sites. Previous studies have shown that mutation of one of the hormone-dependent phosphorylation sites, Ser530, to alanine decreases the transcriptional activity of the receptor under conditions where ligand is limited. Here, we present evidence for the functional significance of another phosphorylation site, Ser211. Mutation of Ser211 to alanine results in a decrease in the transcriptional activity of the receptor and affects the phosphorylation-dependent decrease in mobility of the receptor in SDS-polyacrylamide gel electrophoresis. The degree of reduction in transcriptional activity is dependent on both the cell type and the reporters used in the studies but is independent of hormone concentration, suggesting that phosphorylation at Ser211 regulates the activity of the receptor through a mechanism distinct from Ser530 phosphorylation.